1. Technical Field of the Invention
The present invention relates to a speaker system with broad directivity which can achieve broad band directivity ranging from a low frequency to a relatively high frequency.
2. Description of the Related Art
In a speaker system mounted to a ceiling or the like, a cone type speaker is generally used in a unitary body. In order to increase electric-acoustic transducing efficiency, there may be a demand of desiring to use the cone type speaker with large diameter (over about 6.5 inch or 16.5 cm). However, as the diameter of the speaker is larger, a problem of directivity arises.
FIGS. 7(a) and 7(b) show a feature of a conventional cone type speaker with large diameter.
FIG. 7(a) is a graph showing a directivity when a conventional cone type speaker with large diameter is used. FIG. 7(b) is a graph showing a frequency response when a high frequency speaker is combined with a conventional cone type speaker with large diameter.
In FIG. 7(a), a sound pressure level at a position spaced apart (e.g., 1 m) from the cone type speaker with large diameter along a central axis (0°) of the speaker is set as a reference. When a frequency (f) of sound is in a low frequency range, there exists no directivity. But, in the range from a mid frequency to a high frequency, the sound pressure near the central axis is high, but the sound pressure in an oblique direction is decreased. Thus, when the cone type speaker with large diameter is mounted to a ceiling, the sound is clear right under the cone type speaker, but as being distant therefrom, the sound becomes hollow and unclear.
Thus, when the cone type speaker is used for call announcement or playing music, if it is needed to broaden the directivity for mid and high frequency, the cone type speaker with small diameter is used. However, the transducing efficiency is deteriorated.
Because an equivalent mass of the cone type speaker with large diameter is originally large, an emitting energy of high frequency is not generated. Thus, in order to output the high frequency sound, a high frequency speaker (tweeter) is commonly used together. Because a diameter of the high frequency speaker is small, the high frequency speaker originally has a broad directivity. And, because a size of the high frequency speaker is compact, plural high frequency speakers can be installed.
However, when the cone type speaker with large diameter is used as a woofer and the high frequency speaker is set to output the sound of high frequency higher than a crossover frequency, no problem occurs at the central axis (0°), but a sound pressure in the oblique direction is insufficient in a little lower range than the crossover frequency, as shown in FIG. 7(b). Thus, to combine the high frequency speaker only with the cone type speaker with large diameter has a defect that the broad directivity cannot be achieved with respect to the relatively high frequency sound rather than the low frequency sound.
As an example of a conventional speaker system using the cone type speaker, the speaker system in which a front surface of a cone type vibration plate is covered with a refining member is disclosed for example in Japanese Patent Laid-open Publication No. 2004-193749 and corresponding U.S. Pat. No. 4,554,414. The refining member includes a small diameter center hole and plural peripheral holes around the center hole. When emitting the high frequency sound, the sound pressure level in the front direction is decreased by interference due to phase difference between the sound radiated from the center hole and the sound radiated from the peripheral holes, thereby increasing the high frequency directivity. Also, a diffuser is mounted at the front of the center hole. The diffuser has a substantially conical shape at an upper-half portion, a substantially hemispheric shape at a lower-half portion, and an approximately same diameter as the center hole. Such a diffuser increases the directivity, especially, of high frequency.
As describe above, the conventional speaker system is configured such that a closed space is formed by the cone type vibration plate and the refining member, and the phase interference is generated by radiating the sound energy generated at the closed space from the center hole and the peripheral holes. However, because the closed space constitutes a low-pass filter, the output sound pressure level of high frequency has been decreased.
Further, because the phase interference is performed by using two acoustic paths, i.e., the center hole and the peripheral holes, a peak and a dip of the phase interference become steep.